Recently, in the field of photopolymerization, a research on a cationic polymerization, instead of a radical polymerization, has been promoted to make polymerization easy even in the air without the effect of oxygen.
A cationic polymerization mainly uses as light source a high pressure mercury lamp or a metal halide lamp, including, for example, g-line (436 nm) and i-line (365 nm), and is widely known as a polymerization method for such as an epoxy compound and a vinyl ether compound, rather than a vinyl monomer.
As a cationic photopolymerization initiator, for example, sulfonium salt such as triarylsulfonium hexafluoroantimonate (see U.S. Pat. No. 4,058,401) and a 4-(phenylthio)phenyldiphenylsulfonium salt compound (see U.S. Pat. No. 4,173,476), and an iodonium salt such as diphenyliodonium hexafluorophosphate and diphenyliodonium hexafluoroantimonate (see JP-A-50-151996, JP-A-60-47029, etc.) have been known.
These compounds, however, have such problems of difficulty in preparing a polymer with high hardness when the said compounds are used as a cationic polymerization initiators, because use of a high pressure mercury lamp or a metal halide lamp as light source causes low acid generation efficiency.
Further, these sulfonium salts and onium salts are known to significantly reduce photocuring, when an inorganic strong acid such as hexafluorophosphate (PF6−) is used as a counter anion, compared with hexafluoroantimonate (SbF6−). However, use of SbF6− may be inhibited in the future due to having strong toxicity.
Furthermore, Polish J. Chem., 71, p. 1236-1245 (1997) discloses 2-(phenyliodonio)xanthene-9-one tetrafluoroborate (BF4−) having a xanthonyl group at the cation moiety of the iodonium salt, and a synthesis example thereof. However, there is no disclosure that this compound can be used as a cationic polymerization initiator or not, and use of said compound as a cationic polymerization initiator could not obtain a polymer with sufficient hardness.
On the other hand, a high pressure mercury lamp or a metal halide lamp is widely used as exposure light source for such as a semiconductor resist, a liquid crystal resist, a solder resist for circuit board, PS (Pre-sensitized) plate and CTP (Computer To Plate) plate, and a sulfonium salt and an iodonium salt are also used as an acid generator for those applications.
However, these compounds have such problems that a resist with sufficiently high sensitivity cannot be provided due to low acid generation efficiency, when such as a high pressure mercury lamp or a metal halide lamp is used as light source.
Therefore, sulfonium salts with thioxanthone structure have been developed to provide high acid generation efficiency (see, for example, JP-A-8-165290, JP-A-9-12614, JP-A-9-12615, JP-A-10-60098, JP-A-10-67812, JP-A-10-101718, JP-A-10-120766, JP-A-10-130363, JP-A-10-152554, JP-A-10-168160, JP-A-10-182634, JP-A-10-182711, JP-A-10-279616, JP-A-11-269169 and JP-A-11-322944). However, because these sulfonium salts have absorption in the visible light region not shorter than 400 nm, and therefore show yellowish color. Thus use of these sulfonium salts as a polymerization initiator has such drawbacks that an obtained polymer has color under the influence of hue of said polymerization initiator itself, and therefore use of the said polymerization initiator as coating agents, adhesives or paints causes an obtained polymer with poor transparency and with hue which is different from desired hue.
Under the circumstance, development of an onium salt, providing sufficient hardening function even though PF6− is used as a counter anion, and providing little effect on transparency of an obtained polymer, is required by research on a cation moiety with new structure providing high acid generation efficiency, even when such as a high pressure mercury lamp or a metal halide lamp is used as light source.